Contribution of fallow periods between rice crops to seasonal GHG emissions: effect of water and tillage management

ABSTRACT

Irrigated rice cultivation is a major source of greenhouse gas (GHG) emissions from agriculture. Methane (CH₄) and nitrous oxide (N₂O) are emitted not only throughout the growing season but also in the fallow period between crops. A study was conducted for two transition periods between rice crops (dry to wet season transition and wet to dry season transition) in the Philippines to investigate the effect of water and tillage management on CH₄ and N₂O emissions as well as on soil nitrate and ammonium. Management treatments between rice crops included (1) continuous flooding (F), (2) soil drying (D), (3) soil drying with aerobic tillage (D + T), and (4) soil drying and wetting (D + W). The static closed chamber method was used to measure CH₄ and N₂O fluxes.

Soil nitrate accumulated and N₂O was emitted in treatments with soil drying. Nitrate disappeared while ammonium gradually increased after the soil was flooded during land preparation, indicating net nitrogen mineralization. N₂O emissions were highest in both transition periods in D + W (437 and 645 µg N₂O m⁻² h⁻¹). Methane emissions were significant in only the F treatment. The highest global warming potential (GWP) in the transition between rice crops occurred in F, with CH₄ contributing almost 100% to the GWP. The GWP from other treatments was lower than F, with about 60–99% of the GWP attributed to N₂O emissions in treatments with soil drying. The GWP in the transition between rice crops represented up to 26% of the total GWP from harvest to harvest. This study demonstrates that the transition period can be an important source of GHG emissions with relative importance of CH₄ and N₂O depending on the soil water regime. Therefore, the transition period should not be disregarded when estimating GHG emissions for rice cropping systems.

KEY WORDS:

• Rice
• fallow
• methane
• nitrous oxide
• soil nitrate

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Corrigendum

1. Introduction

Rice is the most important cereal food crop in the world, feeding about 3 billion of the world’s population and supplying 27% of the necessary calories in low- and lower middle-income countries (Dawe et al. 2010; Parry and Hawkesford 2010). In rice production, greenhouse gases (GHG) such as methane (CH₄) and nitrous oxide (N₂O) are produced and emitted. The emission of GHGs into the atmosphere, especially in excessive amounts, has been found to contribute to global warming. It has been documented that 60% of the total global CH₄ emissions comes from biogenic sources, with rice cultivation accounting for 15% (Horwath 2011). N₂O is very potent in reflecting infrared radiation back to the lower atmosphere as it has 298 times higher global warming potential (GWP) than carbon dioxide (CO₂). It contributes around 7.9% of the total GHG emissions (adjusted to CO₂ equivalents) (IPCC 2007).

Irrigated rice, characterized by growing rice on puddled soil with bunds around the field to impound irrigation water, accounts for 75% of the global rice production. This cultivation method also accounts for about 70–80% of CH₄ emissions produced from rice production (Wassman et al. 2000). This is a very intensive system whereby two or three crops of rice can be grown each year, with a fallow period lasting from several days up to 3 months between crops.

During the fallow or transition period between rice crops, the soil can undergo various water and tillage regimes depending on the farmer’s choice of management strategy. In some cases, the field remains flooded especially when water scarcity is not a problem. Under other conditions, however, the field is left to dry until land preparations for the next crop, or it can undergo some tillage while under fallow. Also, the field goes through drying and wetting conditions when rains occur during the fallow period.

The rice soil between crops provides an environment that can lead to various soil chemical transformation processes, affecting the availability of different nutrients and also in the emission of CH₄ and N₂O. Soil water and nutrient availability are primary factors in the production, consumption, and emission of CH₄ and N₂O (Mosier et al. 2004). Different soil conditions result in varying soil redox potential, which is one of the factors that cause different chemical transformation processes (Patrick and DeLaune 1972; Kirk 2004; Beebout et al. 2009). CH₄ and N₂O are emitted from the soil at varying redox potentials, from negative redox favoring CH₄ emissions to positive redox promoting N₂O emissions (Masscheleyn, DeLauna and Patrick 1993). When a field is always flooded, the oxygen supply in the soil is depleted, creating an anaerobic condition which results in a negative redox potential. This water regime allows rice residues and organic matter to decompose, although the rate of decomposition is slower than under well-watered aerobic conditions (Ladha et al. 2011). In addition, very low redox potentials promote the anaerobic fermentation of organic matter in which CH₄ is one of the end products (Neue 1993; Mosier et al. 2004).

In contrast, when an intensively cultivated rice field is allowed to dry during the fallow period, the physical, chemical, and microbiological properties of the soil can be maintained (Kundu and Ladha 1995). In addition, the labile soil N pool can be replenished from the more recalcitrant N pools, resulting in increased N mineralization of the stable N pools. When soil is dried early in the fallow period with some shallow tillage, the remaining crop residues are incorporated and soil aeration is enhanced, resulting in the increased availability of N and phosphorus upon re-flooding (Bucher 2001). Soil drying and wetting conditions also promote an N pool that leads to an increase in N mineralization. While drying conditions during the fallow period enhance the fertility of the soil, they make the soil aerobic, thereby increasing the redox potential and favoring the production of N₂O.

Many studies (Wassman et al. 2000; Khosa et al. 2011; Epule et al. 2011) have indicated high CH₄ emissions during the growing season of rice. Yan et al. (2005) showed that the most significant factor affecting CH₄ flux in the rice fields of Asia is the management of water and organic matter. Studies on CH₄ emissions are, however, greater in number than studies on N₂O emission. Most of the studies on CH₄ and N₂O emissions were done during the growing season of the rice crop (Wassman et al. 2000; Towpayroon et al. 2005; Corton et al. 2000; Lu et al. 2000; Ma et al. 2007) and only a few studies measured CH₄ and N₂O emissions between cropping seasons (Xu and Hosen 2010; Bronson et al. 1997; Abao et al. 2000). These studies on GHG emissions during the fallow period analyzed the effects of different management practices imposed during that period. This inadequacy may cause some underestimation when calculating annual fluxes of CH₄ and N₂O in irrigated rice systems. Hence, this study was conducted to investigate the influence of water supply and tillage during fallow periods on CH₄ and N₂O emissions. In addition, the study also looked into how soil water content and soil inorganic N affect CH₄ and N₂O emissions.

2. Materials and methods

2.1. Site and experiment description

Measurements were taken from a field experiment at the Experiment Station of the International Rice Research Institute (IRRI), Los Baños, Philippines (14°10ʹ07.1″N, 121°15ʹ23.9″E), with an elevation of 21 m above mean sea level (msl) and mean yearly rainfall of 1992 mm (2000–2012). The field was managed from 2003 to 2011 as continuous irrigated rice with two rice crops per year. This study was conducted between the 17th and 18th rice crops in this long-term experiment in the dry season to wet season transition period (May–June 2011) and between the 18th and 19th rice crops in the wet season to dry season transition period (October–December 2011). Harvest dates for crop 17 and crop 18 were 25 April 2011 and 5–7 October 2011, respectively. The fields were flooded on 31 May 2011 and 13 December 2011, respectively, and transplanting was done on 21 June 2011 and 5 January 2012, respectively. The soil was characterized as Aquandic Epiaquoll (Soil Survey Staff 1994) with a clay content of 62%. Soil properties include pH = 6.6, Olsen P = 17–18 mg kg⁻¹, exchangeable K = 0.7–0.9 cmol_c kg⁻¹, organic C = 18 g kg⁻¹, total N = 1.3–1.5 g kg⁻¹, and cation exchange capacity of >40 cmol_c kg⁻¹ (Thuy 2004).

Water and tillage management was imposed under conditions where only stubbles remained as aboveground rice residue during the transition period after harvest. The experiment used a randomized complete block design with three replications. The four treatments were (1) continuous flooding (F), (2) soil drying (D), (3) soil drying with aerobic tillage (D + T), and (4) soil drying and wetting (D + W). The F treatment had the soil in the plot always flooded; D allowed the soil to dry during the fallow period; D + T had the soil aerobically tilled twice with air-drying during the fallow period; and D + W allowed the soil to get wet when it rained and then to eventually dry up. The D and D + T treatments were exposed to the actual weather during the day when there was no rainfall but were covered with tents during rainfall events and at night to ensure that the plots always remained dry during the fallow. At the end of the fallow period, all the plots were flooded, plowed, and hydro-tilled for transplanting of the succeeding rice crop.

2.2. Gas flux measurements

Fluxes of CH₄ and N₂O were measured using the static chamber method every 7 days from 3 May to 14 June 2011 (dry–wet transition) and from 13 October to 29 December 2011 (wet–dry transition) starting at 0930 h. In the wet–dry transition, daily fluxes of CH₄ and N₂O were measured for 4 days following flooding for land preparation.

A stainless steel metal base with a dimension of 40 × 22 × 12 cm was inserted about 10 cm into the soil including the root stubbles of two rice hills. These bases were installed at least a day before sample collection and remained in the field during the fallow period. The internal base height and water depth were measured at each gas sampling time.

The gas collection chambers were fabricated from a plastic box with a dimension of 41 × 24 × 11 cm (41 × 24 × 42 cm in the second fallow period). The chambers were equipped with a thermometer, a sampling port with a stopcock, and a vent for pressure equilibration. In order to ensure air mixture inside the chambers, a battery-operated fan was installed in the large chambers while in the small chambers, this was done by simply ‘pumping’ the sampling syringe for 1 min right before taking the sample.

At every sampling event, the gas collection chambers were placed on the trough of the metal bases and sealed with water. Sixty milliliter of headspace air was taken from inside the chambers using plastic syringes fitted with a stopcock at 0, 15, 30, and 45 min after chamber closure. The samples were immediately transferred to 30-mL evacuated vials with gray butyl rubber septum and analyzed with a Shimadzu 14B gas chromatograph (GC) within 1 week. The GC was equipped with ⁶³Ni electron capture detector (ECD) for N₂O analysis and a flame ionization detector (FID) for CH₄ analysis. The columns for the analysis of CH₄ and N₂O were packed with Porapak Q (50–80 mesh) and the carrier gases used for FID and ECD were nitrogen (N₂) and Argon (Ar), respectively. CH₄ and N₂O standard gases were purchased from Matheson Tri-Gas. Sampling modalities in this experiment follow common practices (Sander and Wassmann 2014) and fulfill minimum requirements of good GHG sampling (Butterbach-Bahl et al. 2015).

Flux rates were computed based on the ideal gas law. The measurements taken at 0930 were assumed to represent daily average flux rates. The method detection limit for the 11- and 42-cm chamber heights was 4.5 and 13.1 µg m⁻² h⁻¹, respectively, for N₂O and 0.01 and 0.02 mg m⁻² h⁻¹, respectively, for CH₄. The GWP for the cropping season was calculated using GWP (CH₄) = 25 and GWP (N₂O) = 298 based on a 100-year time horizon (Forster et al. 2007).

2.3. Soil nitrate- and ammonium-N and soil water

On the day of each gas sampling, soil samples from the respective plots were taken at 0–15 cm soil depth using stainless steel core samplers with a diameter of 5.4 cm. Soil cores were taken from each plot and separately analyzed for gravimetric soil water content and extracted with 2N KCl with a soil:extractant ratio of 1:10 (Keeney and Nelson 1982). The soil extracts were colorimetrically analyzed for ammonium-N (Dorich and Nelson 1983; Kempers and Zweers 1986) and nitrate-N following the procedures of Dorich and Nelson (1984). Soil nitrate- and ammonium-N in the top 15-cm soil layer were converted to kg ha⁻¹ using soil bulk densities taken at different times during the measurement period using soil core samplers with 5 cm diameter in a 0–15-cm soil depth. Soil water-filled pore space (WFPS) was calculated from the bulk density and gravimetric water content using relationships mentioned by Linn and Doran (1984).

2.4. Rainfall

Daily rainfall data during the transition periods between rice crops were obtained from the Climate Unit of IRRI.

2.5. Statistical analyses

Data were analyzed using the PROC MIXED procedure of SAS versions 9.1.2 and 9.3 (SAS 9.1 2003). Homogeneity of the variances was tested for measured variables to assure that residual errors have identical variances across all treatment groups. Logarithmic transformation was conducted before analysis of variance for data sets with heterogeneous variances. Back-transformed means are presented in tables and figures.

Repeated measures of analyses of variance across sampling dates were done for the variables using the first-order autoregressive covariance model. Multiple comparisons among treatments were determined by using the Tukey–Kramer method of pair-wise comparisons of least-square means. Test of the statistical significance among treatments was based on a probability level of 5% or lower.

3. Results

3.1. Rainfall conditions during the fallow period

The daily rainfall during the transitions from dry season to wet season and from wet to dry season is shown in Fig. 1. The fallow between dry and wet seasons until flooding for land preparation lasted for 36 days and the amount of rainfall during this period was 158 mm. In June and thereafter, more rains came, clearly indicating the start of the wet season.

Figure 1. Daily rainfall in the dry to wet season transition and in the wet to dry season transition.

In the transition from wet season to dry season, there was still a lot of rain occurring for several days at the start of and during the fallow period which lasted for 68 days with a total rainfall of 542 mm. At the onset of the dry season, lesser rain was recorded in January.

3.2. Soil bulk density and soil water conditions

Soil bulk density was measured four times in the dry–wet transition and three times in the wet–dry transition from harvest to transplanting (Table 1). Higher bulk densities were measured in the dry–wet transition than in the wet–dry transition period. In both transition periods, bulk density at each time of measurement showed that the F treatment had the lowest bulk density. The D + W treatment had significantly higher bulk density than the F treatment but lower than the two other drying treatments (D and D + T), which had similar bulk densities at all sampling times. In both transition periods, the bulk density in all treatments increased as the fallow period progressed, but it decreased with flooding and decreased further as the soil was continuously submerged through transplanting.

Table 1. Soil bulk density during the transition periods and after soil flooding.

	Bulk density at 0–15 cm soil depth (g cm^−3)^#
Management during the fallow period	Harvest to 13 days after harvest	During fallow	Flooding to 7 days after flooding^‡	Land preparation and transplanting
	Dry–wet transition
Continuously flooded	0.66c	0.65c	0.65b	0.59c
Soil drying	0.96a	1.05a	0.83a	0.80a
Soil drying and tillage	0.95a	1.07a	0.89a	0.84a
Soil drying and wetting	0.81b	0.90b	0.78a	0.72b
	Wet–dry transition
Continuously flooded	0.61b	0.58c		0.55c
Soil drying	0.82a	1.00a		0.76a
Soil drying and tillage	0.82a	1.03a		0.80a
Soil drying and wetting	0.74a	0.80b		0.65b

No samples were taken in the wet–dry transition at flooding to 7 days after flooding.

^#In a column within a transition period, means followed by a different letter are significantly different according to Tukey–Kramer test at alpha = 0.05.

^‡Values in the wet–dry transition are estimated from the available data of the other samplings.

In both transition periods, gravimetric soil water for the F treatment was highest among the four treatments and was consistently high throughout the fallow period (Fig. 2). Despite being continuously flooded, the gravimetric soil water in the wet–dry transition was, on the average, higher by 15% compared to the dry–wet transition period. After flooding, similar gravimetric soil water was observed for both transition periods in the F treatment. The other three treatments had significantly lower gravimetric soil water compared to F in both transition periods. The D + W treatment had higher gravimetric soil water compared to D and D + T in both periods.

Figure 2. Gravimetric soil water and water-filled pore space in the dry to wet season transition and in the wet to dry season transition. Means within a sampling date followed by a different letter are significantly different according to Tukey–Kramer test at alpha = 0.05.

The WFPS in the different fallow treatments during the two transition periods is also shown in Fig. 2. The highest WFPS, ranging from 93% to full saturation, was again observed in the F treatment given that WFPS is affected by soil bulk density and gravimetric soil water. D and D + T showed significantly lower WFPS levels during most of the fallow. During periods with a lot of rain, D + W showed WFPS levels similar to F but during dry periods, levels were comparable to those of the dry treatments. After flooding, the WFPS of all treatments increased and showed similar levels before transplanting.

3.3. Soil nitrate- and ammonium-N during the transition periods

Soil nitrate-N accumulated in the treatments with soil drying (D and D + T), which was significantly higher compared to the F treatment during the transition periods (Fig. 3). The levels of soil nitrate-N increased as the fallow period progressed, with the highest amount measured in the D treatment before flooding in both transition periods. Specifically, in the dry–wet transition, the highest amount of soil nitrate-N (19 kg NO₃–N ha⁻¹) in the D treatment was threefolds higher than D + T (6 kg NO₃–N ha⁻¹) and about seven times greater than D + W (2.5 kg NO₃–N ha⁻¹). The same magnitude of difference in soil nitrate-N between D and D + T was observed in the wet–dry transition. During this time, no soil nitrate-N was observed in D + W due to higher precipitation. The F treatment did not show any soil nitrate-N in both transition periods. When plots were flooded, soil nitrate-N drastically declined approaching zero.

Figure 3. Soil nitrate-N (a) and ammonium-N (b) levels in the 2011 dry to wet season transition and in the 2011 wet to 2012 dry season transition. Means within a sampling date followed by a different letter are significantly different according to Tukey–Kramer test at alpha = 0.05.

In both transition periods, the levels of soil ammonium-N for all treatments at the start of the fallow were similar (Fig. 3). However, as the fallow period advanced, the soil ammonium-N concentration in the F treatment increased and was significantly higher than in the other treatments which had very low levels throughout the fallow period. After flooding and with land preparation, the ammonium-N levels in all the treatments increased, with the F treatment still having the highest.

3.4. Nitrous oxide and methane fluxes during the fallow period

The four treatments imposed during the fallow period showed significant differences in N₂O fluxes (Fig. 4). On the first day of measurement in the dry–wet transition, the highest N₂O flux (437 µg N₂O m⁻² h⁻¹) was measured in the D + W treatment. The treatments D and D + T had N₂O emissions of 81 and 64 µg N₂O m⁻² h⁻¹, respectively. No N₂O flux was measured in the F treatment on the first day of measurement and throughout the fallow period. In the D + W treatment, a surge of N₂O flux (282 µg N₂O m⁻² h⁻¹) occurred when a dry spell followed some rain (17 May); otherwise, N₂O flux declined when soil was wet. In both the D and D + T treatments, the highest N₂O fluxes were measured on the second sampling but these decreased as the fallow progressed. There were no significant differences in N₂O fluxes between D and D + T. In all treatments, N₂O fluxes after the field was flooded ranged from very low to non-detectable.

Figure 4. Nitrous oxide (a) and methane fluxes as affected by tillage management in the 2011 dry to wet season transition and in the 2011 wet to 2012 dry season transition. Means within a sampling date followed by a different letter are significantly different according to Tukey–Kramer test at alpha = 0.05.

In the wet–dry transition, which is longer, similar trends were observed as in the previous fallow period. The F treatment showed mostly fluxes below the minimum detection limit. The highest flux (645 µg N₂O m⁻² h⁻¹) was again measured in the D + W plots on the third sampling, which was 21 days into the fallow, when very little or no rain came in the past 13 days, but rain occurred in the morning of 27 October which was the sampling day. A similar observation was made on 1 December. In the D and D + T treatments, measurable N₂O fluxes were observed; however, they did not exceed 100 µg N₂O m⁻² h⁻¹. When all the plots were flooded and plowed, N₂O fluxes diminished. Daily monitoring of the N₂O flux after flooding indicated some emissions of N₂O in the D, D + T, and D + W treatments (105, 76, and 45 µg N₂O m⁻² h⁻¹, respectively) but none in the F treatment. Monitoring of the N₂O flux in the next 4 days showed very minimal to nil values.

Methane fluxes were observed only in the F treatment in both transition periods (Fig. 4) with the magnitude being smaller in the dry–wet transition than in the wet–dry transition. The highest CH₄ flux in F was 4 mg CH₄ m⁻² h⁻¹ while it ranged from 4 to 13 mg CH₄ m⁻² h⁻¹ in the wet–dry transition. One day after flooding the plots, the highest CH₄ flux of 21 mg CH₄ m⁻² h⁻¹ was measured in the F treatment and still none in the other treatments. However, the CH₄ flux in F decreased thereafter.

3.5. Global warming potentialGWP ¹

The treatments that underwent complete drying during the fallow period (D and D + T) showed the lowest GWP (g CO₂-equivalents m⁻²) (Table 2). Drying and wetting conditions in the fallow period showed a significantly reduced GWP but was higher than the GWP in the D and D + T treatments. The F treatment had the highest GWP.

Table 2. Cumulative global warming potential during the fallow and from soil flooding to transplanting.

	Global warming potential (g CO2-equivalents m^−2)^§#
	Fallow: Harvest to soil flooding^†			Soil flooding to transplanting^‡
Management during the fallow period	Methane	Nitrous oxide	Total	Methane	Nitrous oxide	Total
	Dry–wet transition
Continuously flooded	48.6a	−0.3a	48.3a	14.7a	−0.2a	14.5a
Soil drying	−0.2b	30.5bc	30.3ab	0.6b	0.3a	0.9b
Soil drying and tillage	0.1b	13.9ac	14.0b	3.0bc	0.5a	3.5bc
Soil drying and wetting	0.5b	41.9b	42.4a	4.9c	0.4a	5.3c
	Wet–dry transition
Continuously flooded	332.2a	−0.8a	331.4a	26.7a	−0.5a	26.2a
Soil drying	5.4b	7.9a	13.3b	0.3b	2.3b	2.6b
Soil drying and tillage	11.7b	17.4a	29.1b	1.1b	2.4b	3.5b
Soil drying and wetting	15.5b	49.9b	65.4b	1.1b	1.6b	2.7b

^§IPCC GWP values used for methane and nitrous oxide are 25 and 298, respectively, on a time horizon of 100 years following the 4th assessment report (Forster et al. 2007).

^†The fallow period was 36 days in the dry–wet transition and 68 days in the wet–dry transition.

^‡Soil flooding to transplanting was 21 days in the dry–wet transition and 23 days in the wet–dry transition.

^#In a column within a transition period, means followed by a different letter are significantly different according to Tukey–Kramer test at alpha = 0.05.

During the fallow in the dry–wet transition, the highest total GWP (48.3 g CO₂-eq m⁻²) was obtained from F which is not significantly different from the GWP obtained from the D + W and D treatments (42.4 and 30.3 g CO₂-eq m⁻², respectively). D + T had the lowest GWP (14 g CO₂-eq m⁻²). In the F treatment, CH₄ emission was mainly responsible for the high GWP, while in the three treatments that underwent drying periods in the fallow, the GWP was mainly attributable to N₂O emissions. In the wet–dry fallow, F also had the significantly highest GWP (331.4 g CO₂-eq m⁻²) which was about seven times the GWP in the dry–wet transition and only about twice as long in the wet–dry fallow. Total fallow GWP from the other treatments was significantly lower than F, with about 60–99% of the GWP attributed to N₂O emissions.

Between soil flooding and transplanting, total GWP from F was the highest in both the dry–wet and the wet–dry transition periods with 14.5 and 26.2 g CO₂-eq m⁻², respectively, and with CH₄ contributing 100% to the total GWP. The other treatments had a significantly lower total GWP.

An assessment of the contributions of GHG emissions during the transition between crops (as reported in this study) and the rice cropping season (as reported by Sander et al. 2014) to the GWP from harvest to harvest in treatments without residue incorporation for the two seasons is given in Fig. 5. A significant season by fallow management interaction was found during the transition phase (P = 0.01) and during the cropping season (P = 0.003). In both seasons, F had the significantly highest GWP in each phase of the entire cycle (harvest to harvest), but the transition period contributed more to GWP for the DS than the WS. The GWP during the transition phase contributed 4% and 26% to the total GWP of the entire cycle for 2011 WS and 2012 DS, respectively.

Figure 5. Contribution of (a) the transition GWP and cropping cycle GWP and (b) GWP due to CH₄ and N₂O to the overall GWP from harvest to harvest of the different fallow management treatments without crop residue incorporation.

CH₄ was the major GHG responsible for the GWP of F in all phases. Treatments with dry phases in the fallow period (D, D + T, and D + W) showed significantly reduced GWP both during the growth period and transition period (Fig. 5), with CH₄ having the highest contribution during the growth period (Sander et al. 2014) and N₂O having the highest contribution in the fallow period. In these treatments with drying episodes during the fallow period, the N₂O contribution to the total GWP of the entire cycle ranged from 7% to 24% in 2011 WS and 12% to 17% in 2012 DS.

4. Discussion

4.1. Effect of water and tillage on CH₄ and N₂O emissions between crops

Changes in soil physical, biological, and chemical properties during the fallow period as affected by water supply and tillage practices greatly influence the production and emission of CH₄ and N₂O. When the field was continuously flooded, CH₄ emissions were high and there were very little N₂O emissions. This result closely agrees with results obtained by Bronson et al. (1997). Under flooded conditions, the soil is reduced (negative Eh), which favors the anaerobic decomposition of organic matter, benefits the methanogenic population, and results in high CH₄ production (Wang et al. 1993; Masscheleyn et al. 1993; Jiao et al. 2006). When the field was left to dry, either with or without tillage, only N₂O emissions were observed. Drying the soil during fallow leads to higher soil Eh (~100–300 mV) which would be conducive to N₂O production and emission (Beebout et al. 2009; Masscheleyn, DeLauna, and Patrick 1993). Drying and wetting conditions during the fallow resemble natural field situations best since farmers’ fields are exposed to changing weather conditions. Under these conditions, peaks of N₂O fluxes were observed alongside a reduction in CH₄ emission. However, this study did not provide a strong relationship between rainfall, soil nitrate, and N₂O flux. It was recorded in previous studies that rainfall after a dry spell causes a pulsing effect, which results in N₂O production and emission (Bronson et al. 1997; Abao et al. 2000). Our study showed similar features (e.g., 17 May or 27 October) but was not consistent in capturing these events. However, N₂O peaks might have just been missed as sampling was done manually and not automated and, thus, had a low temporal resolution (Fig. 6).

Figure 6. Nitrous oxide and soil nitrate-N (a), and water-filled pore space and rainfall (b) during the fallow periods in the drying and wetting treatment.

The amount of N₂O emitted to the atmosphere depends on the structure and water content of the soil (Smith et al. 2003). If the N₂O formed can diffuse to an aerated pore, then it is emitted to the atmosphere instead of being converted to N₂. However, when the N₂O is produced below the surface of a saturated soil, then it is more prone to being further reduced to N₂ than escaping to the atmosphere. N₂O emissions occur in most instances when WFPS is less than 90%. Under these conditions, more aerated soil pores can be found so that more N₂O will be emitted before being further reduced.

After flooding the soil and at the start of land preparation activities, GHG emissions in all treatments decrease to zero. CH₄ that has been stored in soil pores in the F treatment is emitted in a pulse during field activities as can be observed in the wet–dry transition. This peak was not recorded in the dry–wet transition due to less frequent gas sampling after flooding. When CH₄ builds up again after field preparation activities, it is not immediately emitted during the first days but stored in soil pores instead. This explains why CH₄ emission was reduced in the F plot after field flooding.

In the other treatments with drying episodes in the fallow period, the CH₄ and N₂O emissions were still very low after flooding the soil for land preparation. During the dry period, the soil becomes aerobic, resulting in an increase in soil Eh and inhibition of methanogens. It takes several days after flooding for CH₄ to form and evolve (Zhang et al. 2011). The decrease in soil Eh to conditions favorable for methanogenic activity takes time and the CH₄ produced first fills soil pores before it is emitted to the atmosphere.

4.2. GWP of the fallow periods

Continuous flooding of the soil during the fallow period contributes to a high GWP, primarily from CH₄ emissions, compared to when there are drying periods in the fallow. This shows that water regime is clearly the most important factor influencing the emission of CH₄ and N₂O. In the dry–wet transition, F had a total GWP of 62.8 g CO₂-eq m⁻² which is higher than that of D + W (47.7 g CO₂-eq m⁻²). D and D + T had a total GWP of 31.2 and 17.5 g CO₂-eq m⁻², respectively. Despite the fact that N₂O was the principal GHG emitted in the drying treatments, the GWP was relatively lower compared to that of the continuously flooded treatment. A similar situation was found in the wet–dry transition. The total GWP of the F treatment was five times higher (357.6 g CO₂-eq m⁻²) than the GWP from the D + W treatment (68.1 g CO₂-eq m⁻²). D and D + T had a total GWP of 15.9 and 32.6 g CO₂-eq m⁻², respectively.

Comparing the emissions per day (total GWP per number of days of the transition period), it can be found that D + T and D + W had similar GWP day⁻¹ in the two transition periods: 0.3 and 0.36 g CO₂-eq m⁻² d⁻¹ for D + T and 0.84 and 0.75 g CO₂-eq m⁻² d⁻¹ for D + W during the dry–wet and the wet–dry transitions, respectively. F had much higher emissions per day in the wet–dry than in the dry–wet transition (3.93 vs. 1.1 g CO₂-eq m⁻² d⁻¹) whereas D had lower emissions in the wet–dry than in the dry–wet transition (0.17 vs. 0.54 g CO₂-eq m⁻² d⁻¹).

In relation to a full cropping cycle (combining the following season), the dry–wet transition had contributions to the overall GWP between around 4% for F and D + T and 14% for D (Sander et al. 2014), while for D + W, the contribution was around 8%. In the cropping cycle of the wet–dry transition plus dry season, higher contributions to the total GWP for F, D + T, and D + W (26.3%, 11.7%, and 16.6%, respectively) were noted. D had a lower transition period contribution of 6.7% in the wet–dry transition plus dry season even though the wet–dry transition is longer at 91 days as against the 57-day dry–wet transition.

The effect of tilling the soil during the drying period on the emission of N₂O and hence GWP was not consistent across seasons. Neither in the dry-wet nor in the wet-dry transition, significant differences between D and D + T were found.

The findings in this study are based on GHG data obtained from measuring direct field emissions. A more holistic picture could be derived if also indirect emissions and emissions resulting from field operations would be taken into account, e.g., through a lifecycle assessment. However, such an analysis would be an entirely different study and is not being pursued here.

5. Conclusion

The study demonstrates that flooding the soil in the fallow leads to high CH₄ emissions but little N₂O flux. As the soil undergoes drying, CH₄ emission is reduced, which leads to a reduced total GWP. Furthermore, this study shows that the fallow period contributes considerable amounts of GHGs to the GWP of a full cropping cycle, especially in the case of flooded fields during the fallow. Although there is probably limited scope for managing fallows to reduce gas emissions as the water regime is determined by climate and hydrology, estimation of GHG emissions from full cropping cycles can be improved depending on the amount of rainfall. For the most common wet and dry conditions, factors of 8.4 and 7.5 kg CO₂-eq ha⁻¹ day⁻¹ could be applied when estimating emissions of the dry–wet and the wet–dry transition periods, respectively, based on our measurements. These factors that would need to be verified through further experiments would be higher in very wet conditions and lower in very dry conditions.

Soil drying leads to an accumulation of nitrate; however, it is immediately lost through leaching and denitrification upon flooding. No strong relation between nitrate concentration and N₂O emissions was found in this study. Based on the presented results, tillage did not have much effect. Management practices that reduce retention of residues under submerged conditions will likely help reduce CH₄ emissions.

Acknowledgments

The Kellogg Company provided support for this research through a grant to IRRI: [Grant Number DPPC-2009-116]. The position of B.O. Sander at IRRI was funded by the Federal Ministry for Economic Cooperation and Development, Germany in 2011–2012: [Grant Number 08.7860.3-001.00]. This work was further supported by the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), which is carried out with support from CGIAR Fund Donors and through bilateral funding agreements. For details, please visit https://ccafs.cgiar.org/donors. The views expressed in this document cannot be taken to reflect the official opinions of these organizations. We thank Ms. Sheryll Elaine Rigua and Ms. Elizabeth Gatchalian for assistance with the statistical analyses, Mr. Ceasar Arloo Centeno for assistance with gas analysis, and Mr. Edsel Moscoso for assistance in preparation of some of the figures.

Additional information

Funding

This work was supported by the German Federal Ministry for Economic Cooperation and Development (BMZ): [Grant Number 08.7860.3-001.00];Climate Change, Agriculture and Food Security Program of the CGIAR; The Kellogg Company: [Grant Number DPPC-2009-116].

Notes

1. In this study, we have used the GWP values for CH₄ and N₂O as suggested in the 4th IPCC assessment report (IPCC AR4, Forster et al. 2007) because we compare the GHG emissions during the fallow period with emissions during the growth period described by Sander et al. (2014) who also used GWP values of IPCC AR4.